This invention relates to an improved process for catalytically cracking a petroleum feedstock, particularly petroleum feedstocks having high metals and carbon residue content. In the refining of crude petroleum to final petroleum products, petroleum fractions are often processed through a catalytic cracking process to increase the yield of distillates, gasoline, and other valuable marketable products.
In the catalytic cracking of petroleum feedstocks, the catalysts used are selected from natural catalyst, synthetic catalyst, and high activity catalyst containing zeolites, for example. These catalysts are well known to those in the refining art. When residual petroleum feeds are employed in the catalytic cracking process, carbon formers and metals, principally vanadium and nickel, deposit on the cracking catalyst, reducing the activity and selectivity of the catalyst for cracking petroleum to important useful products and causing further operational problems in the catalytic cracking unit itself.
The metal concentration on the catalyst can be allowed to build to a certain level at which point steps must be taken to reduce the amount of metals contamination on the catalyst. This can be accomplished in several ways, one of which comprises withdrawing a portion of the cracking catalyst from the unit and replacing it with fresh catalyst, thus reducing the overall metals content of the catalyst inventory. With a relatively modest upper limit of metal deposition allowable on the catalyst, i.e., approximately 1% total metals, petroleum feedstocks having high metals content would become very expensive to process in a catalytic cracking unit requiring large amounts of makeup catalyst and the consequential disposal of large quantities of equilibrium catalyst. Equilibrium catalyst is the catalyst in the cracking unit which is assumed to have attained its equilibrium properties of activity, selectivity, etc. through extended use at relatively constant conditions. At equilibrium, the metal content being removed as a deposit on the catalyst is equal to the metals being added in the feed. It has been found that generally a metals content of approximately 20 ppm in the petroleum feedstock results in satisfactory processing with only a modest replacement of catalyst at a tolerable level.
In view of the increasing demand for products of petroleum cracking, utilizing heavier petroleum charges for feedstock has become a necessity. Available feedstocks include petroleum crude oils and residuums or other fractions which are suitable feedstocks except for high metals or high carbon residue content. Such feedstocks can include oil fractions derived from coal, shale, or tar sands.
Much effort and attention has been devoted to removing the metals and reducing carbon residue from these petroleum feedstocks. For example, in U.S. Pat. No. 3,576,737, a method is taught whereby a petroleum residuum is contacted at a temperature between 600.degree. F. and 900.degree. F. and a hydrogen partial pressure between 100 and 3,000 psig with catalyst particles containing about 0.5 to 10 wt. % of vanadium and having a certain average pore diameter greater than 300 angstroms. The patentee goes to great lengths to describe other prior art efforts for removing metallic contaminants from petroleum residuum. U.S. Pat. No. 3,227,645 describes a method for removing metallic contaminants by contacting a residuum at a temperature of 350.degree. F. to 800.degree. F. and pressures of 200 to 3,000 psig with hydrogen in the presence of a sulfur resistant hydrogenation catalyst comprising one or more of the oxides or sulfides of the compounds of Group VI and/or Group VIII metals supported on a carrier, typically a refractory oxide support such as alumina.
Several modes of practicing such processes are also described in the prior art. For example, U.S. Pat. No. 2,689,825 describes the use of freshly ground fines of a catalyst charge removed from a fluid catalytic cracking unit to contact the feed prior to entry of the oil into the fluid catalytic cracker. The catalyst is said to absorb the metallic contaminants and move with the petroleum residuum through the cracker to be discarded prior to the regeneration of the main body of catalysts in the catalytic cracker. U.S. Pat. No. 3,893,911 describes the use of a fluid catalyst in an ebullating bed reactor to absorb the metals on an activated porous alumina oxide catalyst. This catalyst is characterized in that its activity is related to aging in the presence of vanadium to absorb some vanadium on the catalyst giving it higher activity for removal of the metallic contaminants. The catalyst described in U.S. Pat. No. 3,893,911 is regenerated by the burning of carbon from its surface and returning the catalyst to the reaction zone. U.S. Pat. No. 3,876,530 describes a multi-stage process for removing the metal and sulfur from residuum oils by reaction in the presence of hydrogen and a catalyst system. Catalysts are described as containing a Group VI and at least one Group VIII metal in sulfided condition, such as nickel-cobalt-molybdenum on alumina. Many catalyst combinations are described such as cobalt-molybdenum, nickel-tungsten and nickel-molybdenum. A non-cracking alumina support is mandatory for this multi-stage reaction. The aforementioned patents cite other U.S. patents having disclosures pertinent to the technology of removing metals and other contaminants, particularly sulfur and asphaltenes, from petroleum residuum.
However, it is a characteristic of the above-described processes that a fresh catalyst or a specialized catalyst is required or that the process suffers with respect to economic considerations in one aspect or another. While these processes are sufficient to accomplish the technical objective of removing metals from petroleum residuum, they are lacking in many respects in that they fail to utilize readily available and inexpensive catalyst materials while providing a tailor-made petroleum feedstock for a heavy oil catalytic cracking process.
U.S. Pat. No. 2,771,401 discloses a process in which spent fluid catalytic cracking catalyst is contacted wih petroleum feedstock to remove metals prior to charging the feed to a desulfurization unit.
U.S. Pat. No. 3,691,063 describes a hydrocracking process wherein a used fluid catalytic cracking catalyst is employed in a guard chamber to absorb metals and asphaltenes from a hydrocarbon residuum. The hydrocarbon feed from which the absorbed materials have been removed is then sent to a hydrocracker for processing and the the catalyst is regenerated by contact with steam and oxygen for return to the guard chamber.
A process for removing pentane insoluble asphaltenes and metals contained therein is described in U.S. Pat. No. 3,948,756 which involves a mild hydrogenation in the presence of a hydrogenation catalyst prior to a desulfurization step and further processing.
In view of the limited effectiveness of the above-described process, it is an object of this invention to provide an integrated heavy oil cracking process for the cracking of various petroleum feestocks which utilizes equilibrium catalyst withdrawn from the heavy oil cracking step itself in a pretreating zone integrally connected with the process for cracking the petroleum residuum. It is a further object of this invention to provide a process whereby petroleum with varying metals and carbon residue content can be processed in a heavy oil catalytic cracking unit. It is a further object of this invention to provide a heavy oil cracking process which provides a more efficient and flexible process for the processing of various petroleum feedstocks containing high metals and carbon residue.
It is yet a further important object of this invention to provide an integrated heavy oil cracking process which requires less catalyst makeup and a smaller regeneration zone for catalyst for a given petroleum feedstock containing metals and carbon residue.